Topic Area
Meddies
Project Team Member(s)
LTJG Shawn Gallaher, USN
Major Findings
Meddies are isolated, submesoscale, anticyclonic and subsurface eddies formed exiting the Strait of Gibratar Sill where frictional flow along the Iberian Penisula creates anticyclonic eddies. These vortexes flow toward the northwest initially then are picked up in the Canary Current and flow south-southwest. Approximately 8-12 eddies form each year. The eddies are typically 30 - 100km in diameter, 600m thick, and centered at 1100m below the surface. Meddies are characterized by high salinity (.5 - 1psu) and high temperature (2-3° C) anomalies. Translational speed is approximately 2-5cm/sec, and rotate at approximately 15-35cm/sec.
The decay of a meddy depends on its interaction with seamounts (topography), other meddies, and friction. Meddy propagation by Rossby Wave mechanisms is not usually observed because of the small horizontal size of Meddies. Approximately 70% of all meddies interact with the Great Meteor Seamounts, which causes catastrophic decay and destruction of Meddy properties.Those meddies that avoid interaction have been observed to translate across the North Atlantic Ocean, into the Sargasso Sea along the southern stretch of the North Atlantic Gyre. These eddies have been estimated at life spans near 4 to 5 years and even 7 years in some cases. There decay mechanism, away from topography, is friction which is extremely low in the mid-ocean.
Meddies also undergo thermohaline mixing through double diffision while they propagate, and play an important role in the salt and heat distribution of the Mid-Atlantic Ocean making up nearly 25% of the Mid-Atlantic Salt Tongue. Catastrophic destruction by seamounts lead to visible local maximums in the Mid-Atlantic Salt Tongue.
References
Richardson, P.L., et al, Tracking Three Meddies with SOFAR Floats, Journal of Physical Oceanography, 19, 371-383, 1988.
Prater, M.D., and Sanford, T.B., A Meddy off Cape St. Vincent, Part I: Description, Journal of Physical Oceanography, 24, 1572-1586, 1993.
Fratantoni, D. M., Meddy Collisions with Topography, Woods Hole Oceanographic Institution, http://science.whoi.edu/users/dfratantoni/Meddy/meddy_index.html.
Drakos, Nikos, Acoustic effects caused by a rotating lens, Computer
Based Learning Unit, University of Leeds, 1997.